Telephone circuit



J. B. McKlM 2,294,237

TELEPHONE CIRCUIT Filed June 5, 1940 z sheets-sheet 1 Aug., 25, 1942.

Aug. 25, 1942. J. B. McKlM 2,294,237

TELEPHONE CIRCUIT I' Filedl June 5, 1940 2 sheets-sheet 2 m. .gl

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/N VE NTOR J B. Mc /r/M my J ATTORNEY Patented Aug. 25, 1942 I UNTED STATES PATENT GFFICE TELEPHONE CIRCUIT Application June 5, 1940, Serial No. 338,888

11 Claims.

This invention relates to telephone circuits and particularly to operators circuits for use in fourwire telep-hone systems.

In a four-wire telephone system the trunks from other offices or stations may enter a telephone oiiice on a four-wire basis or may enter the office on a two-wire basis and terminate in the windings of hybrid coils. These coil windings serve to connect a balancing network with the two-wire trunks and connect the two-wire lines with the two channels of a four-wire trunk eX- tending through the oiice trunk equipment, repeaters, switches and other equipment of the oce. The latter trunk connection having hybrid coils for connecting two-wire trunks with four-wire lines may be of the character shown in Patent 2,024,592 to J. B. McKim of December 17, 1935. Voice frequency current amplifying repeaters are usually available for each two-wire channel through automatic selecting equipment associated with both the incoming trunk circuit and the outgoing trunk circuit. Repeaters may be selected for the channels of both incoming and outgoing trunks when these trunks are of sufficient length to warrant their use. For some connections the incoming trunk is of sufcient length to warrant the use of repeaters therewith but the outgoing trunk is not of suicient length to warrant the insertion of repeaters. For other connections repeaters are only selected for the outgoing trunk and for other connections they are not selected for either the incoming or the outgoing trunk. It is well known that the voice amplifying repeater used for such lines permits voice frequency current transmission in one direction only and therefore repeaters in both channels of a four-wire system are arranged for transmission in one direction over one two-wire channel and in the other direction over the other two-wire channel. The trunk hybrid coils which connect two-wire trunks to four-wire trunks are arranged for the latter type of transmission and for transmission through the hybrid coil windings associated with both channelsv when repeaters are not inserted in these channels. The transmission in opposite directions over line channels having repeater therein is through different windings or sets of windings in the trunk hybrid coils.

The object of the present invention is to arrange an operators telephone circuit for connection with the four wires of this system to function for talking and listening to either or both of two connected parties in a manner to prevent voice frequency current interferences.

The operators telephone circuit for this system is arranged for connection with both twowire channels through hybrid coil windings bridged across each of the two-wire channels between the points where repeaters are inserted when required. Other hybrid coil windings are associated with the telephone circuit transmitter and receiver equipment and the telephon-e circuit balancing networks.

A feature of the invention is in devising a telephone circuit for a four-wire telephone system arranged to connect an operators telephone set and balancing network with the two-wire channels of a four-wire system through the windings of a hybrid coil combination so arranged that the telephone circuit may be used for monitoring or conversing including listening over either two- Wire channel having repeaters in one or both channels of both the incoming and outgoing trunks, or repeaters in one or both channels of either the incoming or the outgoing trunks with no repeaters in the channels of the alternate trunk, or lines having no repeaters in the channels of either the incoming or the outgoing trunks.

Another and related feature is in connecting an operators telephone circuit to both two-wire channels of a four-wire telephone system and in so arranging the hybrid coil windings of the telephone circuit that voice current power from conversation by a telephone party over one two-wire trunk having a repeater therein creates a current flow in the operators headset circuit and the balancing network in a direction to nullify curl rent ilow from this circuit to the second two-wire channel of the four-wire trunk system to prevent voice frequency interference in the alternate twowire channel.

Another and related feature resides in adjusting the telephone circuit operators equipment and the telephone circuit balancing networks for monitoring, or for conversing with a telephone partly including listening, over either or both two-Wire channels with or without repeaters therein and in having the windings of the hybrid coils so related that power from one two-wire channel is not transmitted in the form of electrical currents over the alternate two-wire channel.

Another and related feature resides in a balancing network for an operators telephone circuit for monitoring or listening during conversation having a telep-hone receiver equivalent to the operators receiver damped by a pad which is equivalent to the human ear.

These and other features will be discussed more fully in the following description.

To illustrate the features of the invention reference may be had to the accompanying drawings in which:

Fig. 1 illustrates a distant calling office or station and a distant called ofce or station associated through two-wire lines and hybrid coils with the equipment of a four-wire toll telephone oii'ice. This equipment includes an incoming trunk and an outgoing trunk with associated connecting switches, an operators position having a telephone circuit therein and link equipment for associating the telephone position equipment with the incoming trunk;

Fig. 2 illustrates a distant oice connected directly over a four-wire trunk line with. the equipment of a telephone oice as illustrated in Fig. 1; l

Fig. 3 is a diagrammatic illustration of the telephone circuit adjusted for talking and listening while talking;

Fig. 4 is a diagrammatic illustration of the same telephone circuit adjusted for monitoring; and

Fig. 5 illustrates a four-wire system arranged for introducing a repeater in one channel only in the incoming and outgoing trunk circuits.

Line connections The upper portion of Fig. 1 illustrates two distant oices or stations interconnected by an incoming trunk at the left, an outgoing trunk at the right and cross-bar switches I I I, I I 2, I I3 and II4 within the automatically operable telephone olce. It may be assumed that the call is initi- A ated by a toll operator in a distant toll oiice who energized the usual incoming trunk signaling equipment for obtaining the services of the cordless toll operator in the automatically operable telephone oice shown. This automatically operable ciiice may be the same as fully disclosed in patent application to G. V. King, J. B. McKim and O. Myers Serial No. 295,010, filed September 15, 1939 issued as Patent 2,236,246, March 25, 1941. As fully disclosed in the aforementioned patent, after appropriate signals have been transmitted in both directions the calling operator in station |00 gives the cordless toll operator the designation of the distant party that is wanted. In toll service this Wanted party may be connected to the equipment in another distant toll office. The cordless toll operator thus energizes the sender and marker associated with her position for selecting an idle outgoing trunk to thev designated oflice shown as oce |30 connected to the telephone exchange over a two-wire trunk line circuit. This oflce may, however, be connected to the selected outgoing trunk over a four-wire trunk line as shown in Fig. 2. After the trunk to this cflice is selected by the marker the equipment in the marker is adjusted to operate switches such as switches III, H2, ||3 and ||4 for connecting the outgoing trunk equipment with the calling incoming trunk equipment.

Unidirectional voice amplifying repeaters are diagrammatically shown in the incoming trunk andthe outgoing trunk. Repeaters |01 and |08 are in the incoming trunk circuit and repeaters II and ||6 are in the outgoing trunk circuit. The use of amplifying repeaters in either the line incoming from a distantoiice or outgoing to a distant oflice depends upon the character of the line. The character of the line is indicated to the control apparatus which automatically selects repeaters for each channel of the four-wire filed September 15, 1939. It wi1l-be found by referring to the above patent application that a link such as diagrammatically disclosed at I3i is energized for automatically selecting repeaters. After the selection of the repeaters a relay such as |09 in the incoming trunk or ||1 in the outgoing trunk is operated by the action of the repeater selecting equipment. The energization of either of these relays opens the contacts which normally exclude repeater circuit paths from the Pchannels of the trunk circuit. In some instances connections will be established which do not have repeaters in either the incoming trunk or the outgoing trunk, in which case the four-wire channels are used in the same manner as a two-wire channel would be used. For example, the voice frequency currents from distant -oflice |00 would pass through the trunk hybrid coils and energy would be transmitted through hybrid coil windings I03A and |04A which would create equal current in windings |03B and |04B and in both channels of the four-wire circuit, thence over contacts I and 2, 3 and 4 of relay |09, cont-acts I and 2, 3 and 4 of relay |I0, contacts of switches III, |I2, |I3 and II4, contacts I and 2, 3 and 4 of relay II1, windings I20B, I'ZiB, |20A and I2IA, over the two-wire trunk to dist-ant olice I 30.

Other connections may have unidirectional repeaters in the incoming trunk,line but not in the outgoing trunk line. This is done when the incoming line is long and the outgoing line is short, for example, where the outgoing line is extended to an oirlce within the area of the toll center. Under this circumstance the voice frequency currents created by a subscriber or telephone operator would extend to the equipment in oii'ice |00 over trunk conductors |0| and |02 through the hybrid coil windings |03A and I04A, thence through hybrid coil winding |0333, repeater |01, contacts I and 2 of relay IIO, over the upper channel switch contacts, conductors |23 and |24, contacts I and 2 of relay II1, hybrid coil winding I 20B, hybrid coil windings I20A and |2|A to an operator or a subscriber connected with oiice |30. The incoming voice frequencies through hybrid coil Winding I04A `extend through hybrid coil winding IIl4B` but are blocked by repeater |08 and are thus not transmitted over this lower channel. The voice frequency currents created by a subscriber or operator connected with oice |30 travel over the alternate channel. This current is transmitted over the two conductors of the trunk through hybrid coil windings I20A and |2| A, thence through hybrid coil winding |2IB over contacts 3 and 4 of relay |I1, conductors |25 and |26, lower channelswitch contacts, conductors |58 and |59, contacts 3 and 4 of relay IIl, repeating coil |08, winding I 04B,

windings |03A and IMA, trunk conductors `IIlIV and I 02 to the distant subscriber or operator connected with the equipment in oiiice |00. The voice frequency currents from the subscriber associated with office |30 are also transmitted through hybrid coil windings I20A and |20B, thence over conductors |14 and |15, contactsV I and 2 of relay ||1, conductors |23 and |24, |56 and |51, contacts I and 2 of relay I|0 to repeater |01. At this point the voice frequency currents are blocked and areV transmitted no further in the direction of oiiice |00. The relation of these current paths with the telephone circuit which is connected to the four wires of the two telephone channels will be discussed later. The above conditions may also apply to a connection when repeaters are inserted in the outgoing trunk line and not in theincoming trunk line. It is apparent that in this case the Voice frequency current ow with relation to the telephone circuit equipment is quite different, which will also be discussed later.

Other telephone connections have repeaters inserted both in the incoming trunk line and in the outgoing trunk line. As previously described, in this case both relays and H1 are operated for inserting the repeaters in both channels of the incoming trunk and the outgoing trunk circuits. For a gain of this character the voice frequency currents are transmitted from oce |00 through hybrid coil windings |03A, I04A and |03B, repeating coil |01, repeating coil II5, hybrid coil windings |20B, |20A and I2 IA to the distant office |30. The voice frequency currents in this direction also extend through hybrid coil winding |04B but are blocked by repeater |08 in the alternate channel. The voice frequency currents transmitted from office |30 are transmitted through hybrid coil windings |20A, |2|A and |2|B, repeating coil IIE and repeating coil |08, hybrid windings |04B, |04A and |03A to the operator or subscn'ber associated with oce |00. These Voice frequency currents are also transmitted through hybrid coil winding |203 but are blocked by the voice frequency repeater in the alternate channel.

Operators telephone circuit All of the above connections are automatically established as previously stated when the cordless toll operator sets control apparatus for completing a telephone connection. As set forth in the aforementioned application to King et al., the initiation of a call from an operator in the distant ofce |00 energizes the equipment in an incoming trunk circuit which automatically energizes a link diagrammatically shown as |3| for associating the incoming trunk circuit through a channel such as |32 with an idle cordless toll operators position. The equipment of the incoming trunk circuit also actuates a signal in this cordless toll operators position to indicate to the operator the type of call entering a channel in the position. A tone is transmitted to this cordless operator to indicate that the equipment in the position and the incoming trunk circuit has been operated and is in readiness for transmission between the operator at the calling station and the cordless position operator. either automatically operated over lead |31 in response to the operation of other cordless position apparatus or is operated manually from key |30 for adjusting the operators telephone circuit from the monitoring condition shown diagrammatically in. Fig. 4 to the talking and listening condition shown diagrammatically in Fig. 3. The cordless position operator then requests the designation of the wanted party and sets a group of keys for operating a control device. During the conversation between the cordless operatorl and the operator in ofce |00 no repeaters have been inserted in the channels regardless of whether a long or short trunk is associated therewith and consequently the trunk circuit paths are as shown through the contacts I, 2, 3 and 4 of relay |09. The voice frequency currents are thus transmitted from office |00 through the hybrid including coils |03 and |04 over both four-Wire channels of the Relay |81 is trunk circuit, leads |4I, |43, |44 and |46 to the operators telephone circuit and from the operators telephone circuit over the same channels to the operator in the distant ofce |00. After the cordless operator has received instructions from the operator in office |00 and has operated her key-set the equipment of this incoming trunk circuit automatically transmits what are known as class signals to the control circuit which indicates to the control circuit the character of the incoming trunk line and associated equipment. At this time a control circuit may operate relay |09 for inserting repeaters in the channels of the four-wire system. This immediately alters the direction of flow of the Voice frequency currents. Thereafter with relay 01 operated the Voice frequency currents from the distant station |00 ow through the hybrid coil winding |03B, repeater |01 to the operators telephone circuit over conductors |4| and |43, contacts and 2 of key |35, conductors |48 and |40 and through hybrid coil winding |6|A of the telephone circuit hybrid. The voice frequency currents from station |00 are blocked in the lower channel by repeater |08. The voice frequency currents created by the cordless position operator ow through transmitter |54, windings IGEA, B and D of transformer |65 and windings |60C and I6 IC of the telephone circuit hybrid. The transmitter circuit includes a local battery supply extending through windings of retardation coil |62. The voice frequency power generated in the hybrid coil Winding |6|A is transmitted over conductors |48 and |40, through contacts I and 2 of key |35, conductors |4| and |43 to repeater |01 where it is blocked. The voice frequency power generated in the hybrid coil winding |00A extends over conductors |50 and 65| through contacts 3 and 4 of key |35, conductors |44 and |46, repeater |08, thence over conductors |54 and |55, trunk hybrid coil winding |04B, windings I 03A and |04A, over trunk conductors |0| and |02 to the operators telephone circuit in distant office |00. This gives the directions of Voice frequency current transmission between the telephone office |00 and the cordless telephone operators position circuit with and without repeaters inserted in the two channels of the incoming trunk circuit.

The outgoing trunk in the right portion of Fig. 1 is selected and connected to the incoming trunk by switches III, ||2, ||3 and ||4 and also connected to the apparatus of the distant ofce or station |30 before any voice frequency currents pass over the channels of this trunk. If repeaters are required in the outgoing trunk, these are also inserted before Voice frequency current passes over the channels. The voice frequency current flow between an operator at station |30 and an operator at station |00 has already been discussed. Conversation between the cordless toll operator and the operator at station |30 is required when difficulty is encountered, when an incorrect connection has been made or sometimes to give the distant operator the designation of a wanted party. Therefore the operators telephone set, Fig. 1, may be associated with the operators equipment in station |30, for talking and listening to the operator in station |00 and for monitoring after a telephone connection is completed over the channels of the four-wire system either having no repeaters in the channels or with repeaters in these channels.

The operators telephone circuit for this fourwire system may be divided into the following parts as diagrammatically shown in Figs. 3 and 4.

1. Monitoring headset including the operators receiver and hybrid coil windings.

2. Monitoring balancing network including a receiver like the operators receiver damped by a pad to simulate the human ear and hybrid coil windings.

3. Talking and listening headset including the operators receiver, transmitter, the telephone set induction coil for coupling the receiver circuit and transmitter circuit and hybrid ccil windings.

4. Talking and listening balancing network including the receiver like the operators receiver damped by a pad to simulate the human ear, a resistance and capacity element to simulate the transmitter and associated equipment, an induction coil for coupling the receiver network to the transmitter network and hybrid `coil windings.

5. A relay |61 for adjusting the telephone circuit from the monitoring condition and 2 to the talking and listening condition 3 and 4 or the reverse, depending upon the wiring connections of this relay. The relay may be wired so that in its normal position the talking and listening headset and balancing network are connected to the four wires of the channels and when operated it associates the monitoring set and network with the four wires of the channels. This relay also adjusts the hybrid coil windings of the tele- -phone circuit which are bridged across the twowire channels so that two windings are bridged across each channel for monitoring and one winding is bridged across each channel for talking and listening. The second winding is added during monitoring to prevent a noticeable change in the transmission or" the parties talking over the telephone lines.

Current flow in telephone circuit and networks The current flow in the operators telephone set during monitoring of the various types of line connections will first be considered. It will be assumed that the telephone set has been associated with the two channels of a line comprising an incoming trunk and an outgoing trunk both of which have voice frequency'repeaters therein. That is, both relays and II? have been operated to include the repeaters in each channel of the incoming trunk and the outgoing trunk. It will also be assumed that voice frequency current is being transmitted from station |00 to station |30. The voice frequency current ilow will pass through hybrid coil windings IBSA and IMA which create a current ow through hybrid coil winding |0313, repeater |01, repeater II5, hybrid coil winding 920B, hybrid coil windings IA and IZIA to the equipment in station I 30. This current also flows to the cordless operators telephone circuit over conductors I4| and |43 through key |35, conductors H53 and |49 through the telephone circuit hybrid coil windings IGIA and IIB. The Voice frequency current flow from station |00 through hybrid coil winding |0413 does not reach the Vtelephone circuit since it is blocked by the unidirectional repeater HBS which only transmits Voice frequency Cil rent flow in a given direction may be assumed 75 to give the effect upon the operators telephone circuit network, the telephone set and the alternate channel. It may therefore be assumed that the current flow is from terminal I of hybrid coil winding I6IB to terminal 2 and continuing through hybrid coil winding ISIA to terminal 3. This creates a current ow in the network in the direction from terminal 5 to terminal 4 of hybrid coil winding IEiID, thence from terminal I4 to terminal |13 of hybrid coil winding |60D through the network balancing receiver |68 to terminal 5 of hybrid coil winding IGID. The current flow in the operators monitoring set is created in a direction from terminal I of hybrid coil winding |6IC to terminal 6, terminal I5 of hybrid coil winding I60C to terminal I6, through the operators receiver |63 to terminal I of hybrid coil winding ISIC. It will be noted that the current flow in the network hybrid coil winding IBDD is in the opposite direction to the current ilow in monitoring winding |600. Equal and opposing fluxes are created in windings I60C and |6033 so that no current ilows through the windings |60A and |60B which are bridged across the channel having repeater |08 therein. If voice frequency current were permitted to flow through windings I 60A and ISB of the hybrid, this voice frequency current from conversation over the upper channel through repeater |01 would return over conductors and I5| through contacts 3 and 4 of key |35, conductors |44 and |45, through repeater |98 and be transmitted in the opposite direction from whence it came, thus causing a noticeable disturbance in the line hybrid which is sometimes known as a Voice frequency echo.

The same monitoring condition would prevail when voice frequency current is transmitted from a distant office or station |30 with repeaters I|5 and IIS in the outgoing trunk channels. In this condition voice frequency current is transmitted through hybrid coil winding I2 IB, over the lower channel, repeater I I6 and thence to the telephone circuit over conductors |44 and |46 through contacts 3 and 4 of key |35, conductors |50 and I5I, through hybrid coil windings |A and |60B.

This is the opposite condition in the telephone circuit from the one previously described. Assuming now a current flow from terminal I2 yof hybrid coil winding |50A to terminal |I and through hybrid coil winding IO0B to terminal I0, a current is created in the network hybrid coil windings |60D and ISID and the telephone set hybrid coil winding ISOC and IIIC. This current may be vassumed to flow from terminal I5 to terminal I6 in hybrid coil winding |600 and from terminal .I3 to terminal I4 in hybrid coil winding |60D. The hybrid coil windings of the network are so related that the current path through the network winding |0|D is from terminal 4 to terminal 5 and in the operators set hybrid coil winding IBIC the path is from terminal to terminal 6. This creates an equal and opposite flux and consequently no current flows through the hybrid coil windings |6IA and I6|B connected with the alternate channel of the fourwire line over conductors I 48 and |49 through contacts I and 2 of key |35, conductors |4I and |43 to the upper channel as shown. Voice frequency current if permitted to flow through hybrid coil windings IGIA and IBIB would, of course, be blocked by repeater |01 but would be transmitted through the repeater |I5 and hybrid coil winding I 20B to create a voice frequency disturbance in the associated channel.

The monitoring condition may now be assumed on a line connection having repeaters inserted in each channel of the incoming trunk circuit by the operation of relay and an outgoing trunk circuit having no repeaters, in which relay ||1 is normal as shown. In this case voice frequency current created by conversation over the trunk from ofice |00 would be transmitted through repeater |01 and over the upper channel to station |30 as described. When monitoring on this conversation the voice frequency current will flow to the operators telephone circuit over conductors |4| and |43 which would create a circuit condition in the monitoring headset and network as previously described so that current will not flow from the telephone circuit hybrid coil combination through repeater |08 to the telephone oce. The condition is different when voice frequency current is created by conversation in ofce |30. This voice frequency current flows through trunk hybrid coils over the lower channel through repeater |08, hybrid coil winding |04B to the distant oiiice |00 and would be blocked in the upper channel by repeater |01. Current flow to the telephone circuit hybrid would, however, be transmitted through trunk hybrid coil windings IB and I2 IB, thence over contacts 2, 3 and 4 of relay |I1, conductors |23, |24, and |26,conductors |4|, |43, |44 and |46, contacts I, 2, 3 and 4 of key |35, conductors |48, |49, |50 and I5| through the telephone circuit hybrid coil windings I60A, |60B, |6|A and I6IB which are bridged across these channels. Voice frequency current is thus created in the telephone circuit which for this example may be assumed to 'ow through hybrid coil windings |60A and |60B from terminal I2 to terminal I0 and in windings |6I A and |6|B from terminal to terminal 3. This current flow is thus in the same direction through the headset circuit from terminal I5 through hybrid coil winding |600 to terminal I6, through the operators receiver |66, terminal 1 of h'ybrid coil winding |6| C to terminal 6, since in this case the current flow through the hybrid coil windings bridged across the two channels enters the telephone circuit hybrid coil windings in opposite directions. The two Voltages in the channel circuits associated with the network are equal and since the network coil windings are equal and opposing in direction with relation to the current flow no current flow is created in the network. Under this condition the network balances the line condition equally as well as under other conditions where voltage is generated therein.

After a telephone connection has been completed the cordless position operator may converse with either lan operator or subscriber in ofce area |00 or an operator or subscriber in oce area by operating relay |61. Relay |61 associates the operators transmitter |64, the operators receiver |66 and the network with both channels of the four-wire system. In Fig. 3 the arrangement is shown diagrammatically with the operators headset equipment for talking and listening in the rectangle 220, and the network for talking and listening in the rectangle 22|. The cordless operator may talk to both parties when the two parties are connected together or may talk to one party or the other by separating the two parties. The parties are separated by the operation of key |34 which associates ground through the winding of relay I I0 to battery. The operation of relays I I0 and |61 associates the operators telephone set with the incoming operator or party associated with oice |00. With key |35 energized office |00 is cut off and the telephone circuit is associated through the inner contacts of relay I|0, switches III, |I2, ||3 and ||4 with the office |30. Voice frequency current from the operators telephone set is transmitted through both' hybrid coil windings |60C and I6 IC. This produces a current flow in windings |60A and |6|A which is transmitted to both channels of the four-wire system. When the cordless toll operator is talking in the direction of station |00, these voice frequency currents are transmitted through repeater |00 and thence over trunk conductors I 0I and |02, but the voice frequency currents through' the hybrid coil winding bridged across the alternate two-wire channel are blocked by repeater |01. If both oiices |00 and |30 are connected together with relay ||0 not operated the latter voice frequency currents would be transmitted through repeater ||5 to the equipment in oiice |30. With key |35 energized and relay |I0 energized, the voice frequency current flow is also transmitted through the telephone circuit hybrid coil windings, through repeater I5 to the distant oiice |30, but the voice frequency currents 'are blocked in the alternate channel by repeater |I6. Under the same condition if no repeaters have been inserted inthe channels the voice frequency current from the hybrid coil windings bridged across the alternate channels is transmitted over both the upper and lower channels through trunk hybrid coil windings I20B and I2IB, trunk hybrid coil windings |20A and IZIA to the distant office. This is also true in talking to the party at station |00 if repeaters |01 and |00 have not been inserted in the two channels of the four-wire system.

When the cordless operator talks to a party in oice |00 or |30 the network shown in the rectangle 22| is arranged to perfectly balance the operators headset circuit. This perfect balance is made possible by the feature of using a receiver |68 which is identical with receiver |66 and a pad |69 which is identical with the pad produced by the human ear against receiver |66.

The current flow conditions in the network 22| are similar to those described for the monitoring network 23|. For example, voice frequency current from station |00 when repeaters have been inserted in the channels, flows through repeater |01 and is blocked by the repeater |08 in the alternate channel. The current flow therefore enters the telephone circuit over conductors |48 and |49 which creates a current ow in the operators headset circuit in a single direction through hybrid coil windings I6IC and |60C. The current flow in the network hybrid coil windings |6ID and |60D is, however, in opposite directions which creates an equal and opposite current flow in the hybrid windings |600 and |60D. No power is created in the hybrid coil winding |60A and thus there is nocurrent flow over the alternate two-wire channel associated with the conductors |50 and |5I. If this current were permitted to flow over these conductors voice frequency current would pass through repeater |08, causing a disturbance in the trunk channel which would be apparent to the party talking from station |00. If no repeaters are in the upper and lower channels when a person is talking from station |00, the voice frequency current flows over both the channels through hybrid coil windings |60A and |6IA bridged across these channels. The current ow through receiver |66 is in the same direction as before, thus creating opposite and equal current flow in the network hybrid coil windings |66D and |6|D. In this case no current ows in the balancing network. Assuming now that relays |67 and I0 are operated and key |35 is operated, .a person talking from station |30.' would create voice frequency current over the lower channel and through repeater H6, assuming that repeaters have been inserted in the outgoing trunk channels. This current is blockedY by repeater H5. Thus the voice frequency current ows over conductors |25and |26 through contacts 3 and 4 of relay ||0, conductors |45 and |41, contacts 3 and 4 of key |35, conductors |50 and |5| through hybrid winding |'60A. Assuming that the current iiow is from terminal |2 to terminal in the hybrid coil winding |60A, this current flow then would be in a direction from terminal l to terminal I6 in hybrid coil winding |600, through the receiver |66 and associated windings of transformer |65, terminal 1 to terminal 6 of hybrid coil winding |6|C. In this case a current is created in the network hybrid coil winding |60D from terminal I3 to terminal I4, terminal 4 to terminal 5 of' hybrid coil winding |6| D, windings of transformerv coil and receiver |68. The current created in hybrid coil windings |6|C and |6|D is thus equal but in opposite directions and does not create any power for current now in hybrid coil Vwinding |6|A associated with the alternate channel. If no repeaters have been inserted in the outgoing trunk channels the voice frequency current flowing from station |30 would pass over both channels through contacts Zand 3, 4 of relay ||1 and thus enterl hybrid' coil windings |60A and |6| A of the telephone circuit. As previously stated, this creates a current flow through hybrid coil windings |60C and |6|C for the operators receiver |66, but creates an equal and opposite power in hybrid coil windings |60D and |6|D and therefore creates no current flow in the network during this ow of voice frequency current from thestation |30.

Fig. 2 illustrates a distant cnice 2|0 connected to the toll telephone oice over a four-wire system. The trunk conductors 203, 204, 205 and 206 are thus directly connected between the two telephone oices. In this case no hybrid coil is required in the telephone cnice for -associating an incoming trunk or an outgoing trunk with the telephone equipment of the oiice. Fig'. 2A has been shown as an outgoing trunk having relay 202 for automatically inserting repeaters in both channels the same as described for relay This trunk Vis associated with an incoming trunk by the energization of switches the same as switches to ||4. A single switch 2|4 has been shown and conductor 2|5 would connect to a switch the same as switch l|3. Voice frequency current from station 2|0 would enter the cnice in the same manner as described when a hybrid is used. The voice'frequency currents from station 2|0 would be transmitted through voice frequency repeater 20| if repeaters are inserted in both channels. The voice frequency current from an cnice such as |00 or from la cordless position operator would be transmitted through voice frequency repeater 200. The two channels of this four-wire system are associated with the cordless operators telephone circuit in the same manner as previously described where hybrid coils are used for-connecting a-two-wire line with the four-wire system in the telephone office andthe eiect of voice frequency currents is the same in the operators telephone circuit The trunk in and reacts in the same manner' as described for the aforementioned four-wire system using line hybrids.

The four-wire system diagrammatically shown in Fig. 5 may be assumed to be the same as shown in Fig. l with the exception that for the sake of economy repeaters are available for only one channel of the incoming and the outgoing trunk circuit. It is clear that the operators telephone circuit herein disclosed functions in the manner described for this character of line.

What is claimed is:

l. In a telephone system, a telephone oice, a trunk incoming to said office having two twowire transmission channels, trunks outgoing from said office, means for establishing a conversational connection .between said incoming trunk and one of said outgoing trunks, voice current repeaters normally disconnected from said trunks, means for connecting said repeaters in said two-wire channels if required, an operators telephone circuit for listening to said conversation, coils forming part of said telephone circuit independent of said channels having windings arranged for separately bridging the two wires of each of said channels, means for connecting said bridging windings to the wires of both of said channels, and means in the telephone cir- -cuit for preventing voice transmission from either channel to the alternate channel through said telephone circuit coils with or without repeaters in said channels.

2. In a telephone system, a telephone ofnce, a trunk incoming to said oice having two-wire transmission channels, trunks outgoing from said cice, means for establishing a conversational connection between said incoming trunk and one of said outgoing trunks, voice repeaters normally disconnected from said trunks, means for connecting said repeaters in said two-wire channels if required, an operators telephone circuit for listening to said conversation, coils forming part of said telephone circuit independent of said channels having windings arranged for separately bridging the two wires of each of said channels, means for connecting said bridging winding to the wires of both of said channels, and means in the telephone circuit including a balancing network cooperating with the bridged winding connected to both channels of said four-wire trunk to prevent voice transmission from either channel to the alternate channel through said telephone circuit with or without repeaters in said channels.

3. In a telephone system, a telephone cnice, a trunk incoming to said office having two two-wire transmission channels, trunks outgoing from said office, means for establishing a conversational connection between said incoming trunks and one of said outgoing trunks, voice current repeaters normally disconnected vfrom said trunks, means for connecting said repeaters in said twowire channels if required, an operators telephone circuit having an operators headset circuit and a balancing network, and means to connect said telephone circuit to both said channels including hybrid coil windings in said headset circuit and network and windings bridged across the wires of each channel cooperating with the channels of said four-wire trunk to prevent Voice transmission from either channel to the alternate channel through the telephone circuit.

4. In a telephone system, a telephone oiiice, trunks incoming to said office having two-wire transmission channels, trunks outgoing from said office having two two-wire transmission channels, means for establishing conversational connections between said incoming trunks and said outgoing trunks, voice current repeaters normally disconnected from said trunks, means for connecting said repeaters in any two-wire channels of the connected trunks if required, an operators telephone circuit common to said trunk circuits and independent thereof, means for connecting said telephone circuit to the two wires of both channels of said trunks, and means in said telephone circuit for preventing voice transmission from either channel to the alternate channel through said telephone circuit with or without repeaters in the channels as required.

5. In a telephone system, a telephone office, trunks incoming to said office having two twowire transmission channels, trunks outgoing from said oice having two two-wire transmission channels, means for establishing conversational connections between said incoming trunks and said outgoing trunks, voice current amplifying repeaters for unidirectional transmission over said channels normally disconnected from said trunks, means for connecting said repeaters in any twowire channels of the connected trunks if required, an operators telephone circuit common to said trunk `circuits and independent thereof having an operators heads-etcircuit and a balancing network, and means to connect said telephone circuit to both said channels between the points where the repeaters are inserted in the incoming and outgoing channels, inclu-ding hybrid coil windings in said headset circuit and network cooperating with associated hybrid coil windings of said telephone circuit bridged across the wires of each channel to prevent voice transmission from either channel to the alternate channel through said hybrid coil windings, either with or without repeaters in said channels.

6. In a telephone system, a telephone oliice. trunks incoming to and outgoing from said office having two two-wire transmission channels, means for establishing conversational connections between said incoming and outgoing trunks, voice current repeaters normally disconnected from said trunks, means for connecting said repeaters in any two-wire channel of the connected trunks if required, an operators telephone circuit common to said trunk circuits and independent thereof including a hybrid coil combination formed into the operators headset circuit, a line balancing network and channel bridging circuits, means for connecting said channel bridging circuits to the wires of both channels, means for altering the connections of said headset circuit and network for talking and listening and for monitoring on a conversation over said fourwire trunk connection, the hybrid coil combination in said telephone circuit being so arranged as to prevent voice transmission from either channel to the alternate channel through said telephone circuit when connected for talking and listening or monitoring and with or without repeaters in said channels.

7. In a telephone system, a telephone ofce, trunks incoming to and outgoing from said ofce having two two-wire transmission channels, means for establishing conversational connections between said incoming trunks and said outgoing trunks, unidirectional voice current repeaters normally disconnected from said trunks, means for connecting said repeaters in any twowire channel of the connected trunks if required,

a telephone circuit common to said trunk cirfil cuits and independent thereof including a hybrid coil combination formed into the operators headset circuit, a line balancing network and `channel bridging circuits, means for connecting said headset and network through said hybrid coil windings with both channels of said connected trunks for listening to conversations in both directions, and means for connecting said telephone circuit through hybrid coil windings with both channels of either the incoming trunk or the outgoing trunk independent of the other for separately listening to conversations from either direction, said conversations over one channel creating voice currents in the telephone circuit hybrid coil windings in a direction to prevent voice current disturbances in the alternate channel.

8. In a telephone system, a telephone office, trunks incoming to and outgoing from said offlce, means for establishing a conversational connection between an incoming trunk and an outgoing trunk, an operators telephone circuit having an operators telephone headset circuit including a receiver and a balancing network having an identical receiver and a pad for said receiver simulating the pad produced by the human ear pressed against said receiver, and means for connecting said operators telephone circuit with said trunks.

9. In a telephone system, a telephone oiiice, trunks incoming to and outgoing from said oflice having two two-wire transmission channels, means for establishing conversational connections between said incoming trunks and said outgoing trunks, voice current amplifying repeaters for unidirectional transmission over said channels normally disconnected from said trunks, means for connecting said repeaters in any two-wire channels of the connected trunks, an operators telephone circuit having an operators telephone headset circuit and a balancing network, means including a hybrid coil combination for connecting said telephone circuit to the two wires of both channels through windings separately bridged to alternate channels, windings in said headset circuit and windings in said balancing network equal to the windings in said headset circuit for the transmission of voice currents through one bridged winding from its associated channel to the headset circuit and the balan-cing network, which creates current in the same direction through the hybrid coil windings of the headset and network associated with the channel over which the voice currents were transmitted but in opposite directions through the hybrid coil windings of the headset and network cooperatively associated with the hybrid coil winding bridged to the alternate channel which prevents the creation of electrical power in said latter bridged winding and prevents current flow in said alternate channel.

10. In a telephone system, a telephone oice, trunks incoming to and outgoing from said office having two two-wire transmission channels, means for establishing conversational connections between said incoming trunks and said outgoing trunks, voice current repeaters normally disconnected from said trunks, means for connecting said repeaters in said two-wire channels 4as required, an operators telephone circuit for listening to conversations over said channels and for talking over said channels comprising a hybrid coil combination formed into a headset circuit, a network arranged to balance the lines for either listening or talking over sai-d lines and separate bridging windings for connection to the two wires of each channel, so-arranged that current from one channel passing through one bridged winding creates current in the heads-et and network in such a -direction that no current passes to the alternate bridged winding and that current from both channels passing through both bridged Windings is neutralized in the network and thus creates no current flowing in said network.

' 11. In a telephone system, a telephone office, trunks incoming to and outgoing from said oliice having two two-wire transmission channels, means for establishing conversational connections between said incoming trunks and said outgoing trunks, voice current repeaters normally disconnected from said trunks, means for connecting said repeaters in the two-wire channels of said trunks as required, said trunks being so arranged that voice currents incoming from a station flow equally over both channels of a first trunk to the other connected trunk with the repeaters normally disconnected and flow over a single channel of the rst trunk to the other connected trunk with repeaters connected in the two-wire channels of said rst trunk, an operators telephone circuit forl listening to conversations over said channels and for talking over said channels, comprising a hybrid coil combination formed into said telephone circuit having windings bridging the two wires of each channel between connected trunks so that voice currents iiowing over both channels from a rst trunk will iiow equally through both said bridged win-dings and voice currents flowing over one channel will iiow through only one bridge winding, said hybrid coil combination also being forme-d into a headset circuit and network of the operators telephone circuit electrically coupled to said bridged windings, so arranged that voice currents from one channel passing through one bridged winding creates current in the headset circuit windings and network windings which is aiding in the windings coupled to said one bridged winding and opposed in the windings coupled to the bridged winding connected to the alternate channel to prevent voice transmission through sai-d telephone circuit to said alternate bridged winding and said alternate channel and so arranged that voice currents from both channels passing through both bridged windings creates aiding current in the headset circuit windings coupled to both bridge-d windings and equal and opposite voltages in the network windings so that no current iiows in the network thus preventing voice transmission through said telephone circuit to either channel. Y

JAMES B. McKIM. 

